Use clang toolset by default as we have no gcc in main.

--- a/tools/build/src/build-system.jam
+++ b/tools/build/src/build-system.jam
@@ -633,7 +633,7 @@ local rule should-clean-project ( project )
         }
         else
         {
-            default-toolset = gcc ;
+            default-toolset = clang ;
             if [ os.name ] = NT
             {
                 default-toolset = msvc ;
--- a/tools/build/test/BoostBuild.py
+++ b/tools/build/test/BoostBuild.py
@@ -99,7 +99,7 @@ def get_toolset():
     for arg in sys.argv[1:]:
         if not arg.startswith("-"):
             toolset = arg
-    return toolset or "gcc"
+    return toolset or "clang"
 
 
 # Detect the host OS.
@@ -276,7 +276,7 @@ class Tester(TestCmd.TestCmd):
         self.pass_toolset = pass_toolset
         self.ignore_toolset_requirements = ignore_toolset_requirements
 
-        prepare_prefixes_and_suffixes(pass_toolset and self.toolset or "gcc")
+        prepare_prefixes_and_suffixes(pass_toolset and self.toolset or "clang")
 
         use_default_bjam = "--default-bjam" in sys.argv
 
